This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2003-310681 filed in Japan on Sep. 2, 2003, the entire contents of which are hereby incorporated by reference.
The present invention relates to a velocimeter which irradiates laser light onto a traveling measured object and receives scattered light of an optical frequency shift quantity corresponding to the travel velocity of the measured object from the measured object to detect the velocity of the measured object, a displacement information measurement device which calculates displacement information of the measured object based on velocity information and time information of the measured object, and a conveying and processing device which controls conveying of the measured object based on the velocity information or displacement information and applies a prescribed process to the measured object.
When a light source and an observer move relative to each other, light is generally subjected to frequency changes by the Doppler effect. The laser Doppler velocimeter (hereinafter, referred to as LDV) uses this effect to measure a speed of a measured object by irradiating laser light onto the moving object and measuring a Doppler frequency shift of the scattered light from the measured object. This laser Doppler velocimeter, which was disclosed by Yeh and Cummins in 1964 (Appl. Phys. Lett. 4-10 (1964) 176), is widely known and is in practical use today.
FIG. 13 shows an optical system view of a conventionally typical LDV.
In FIG. 13, reference numeral 101 denotes a laser diode (hereinafter, referred to as LD) as a semiconductor laser, 102 denotes a photodiode as a photodetector (hereinafter, referred to as PD), 103 denotes a diffraction grating, 104 denotes a collimator lens (hereinafter, referred to as CL), 105 denotes a mirror, 106 denotes a condenser lens, 107 denotes a first light flux of +1st order diffracted light by the diffraction grating 103, 108 denotes a second light flux of −1st order diffracted light by the diffraction grating 103, and 113 denotes a measured object.
In the optical system as constituted above, laser light emitted from the LD 101 is converted by the CL 104 into a parallel light flux, and then is split into ±1st order diffracted lights at a diffraction angle of θ by the diffraction grating 103 to become the first light flux 107 and the second light flux 108. The first light flux 107 and the second light flux 108 are respectively reflected by the mirrors 105 and are then made incident on a surface of the measured object 113 at an incident angle of θ to be overlapped each other again. The first light flux 107 and the second light flux 108 scattered by the measured object 113, which are Doppler frequency-shifted, are slightly different from the LD 101 in oscillating frequency. As a result, the interfacial waves of the first light flux 107 and the second light flux 108 scattered by the measured object 113 generate beat. This beat is termed beat signal. The speed of the measured object 113 is obtained by heterodyne-detecting a beat frequency of the beat signal using the PD 102. Hereinafter, this will be explained in further detail.
Here, when a direction in which the measured object 113 moves to the right as shown in FIG. 13 is set as a forward (positive) direction, the frequency shift of the first light flux 107 is Doppler frequency-shifted by −fd and the second light flux 108 is Doppler frequency-shifted by +fd, so that an apparent frequency of the first light flux 107 becomes (f0−fd) and an apparent frequency of the second light flux 108 becomes (f0+fd). Note that f0 represents the oscillating frequency of the LD 101. In this case, since an electric field of the light emitted from the LD 101 is represented as E0·cos (2πf0t) wherein E0 denotes an amplitude thereof and t denotes time, the first light flux 107 is indicated by the following Equation (1) and the second light flux 108 by the following Equation (2):IA=EA·cos{2π(f0−fd)t+φA}  (1)IB=EB·cos{2π(f0+fd)t+φB}  (2)wherein f0 denotes a frequency of a beam outgoing from the LD 101, E0 denotes an amplitude of the beam outgoing from the LD 101, EA denotes an amplitude of the first light flux 107, EB denotes an amplitude of the second light flux 108, φA denotes a phase of the first light flux 107 and φB denotes a phase of the second light flux 108.
Since the frequency of light is generally 100 THz (up to 1014 Hz), it is impossible to measure the frequency information of Equation (1) and Equation (2) directly. Therefore, a heterodyne detection is generally employed, and since f0>>fd is established, an interfacial wave of Equation (1) and Equation (2) is indicated by the following equation:
                              〈                                                                                    I                  A                                +                                  I                  B                                                                    2                    〉                =                                                            E                A                2                            +                              E                B                2                                      2                    +                                    E              A                        ·                          E              B                        ·                          cos              (                                                2                  ⁢                                      π                    ⁡                                          (                                              2                        ⁢                                                  f                          d                                                                    )                                                        ⁢                  t                                -                                  (                                                            ϕ                      A                                        -                                          ϕ                      B                                                        )                                            }                                                          (        3        )            Note that < > in the left side of Equation (3) represents time average. Consequently, the PD 102 allows the frequency of the interfacial wave to be measured.
FIG. 14 shows a case when as the measured object 113 moves at a speed of V, two light fluxes are made incident on the object 113 at arbitrary angles of α and β respectively, and an observer receives scattered light at an arbitrary angle of Υ.
Frequency shift quantity due to the Doppler effect, which is obtained using the Lorentz transformation based on relativism in a precise sense, is approximately obtained as follows when the moving speed V is sufficiently smaller than speed of light c. Relative velocities VA1 and VB1 of the moving object 113 with respect to lights from light sources A and B are indicated by the following equations:VA1=c−V sin αVB1=c+V sin β  (4)Also, apparent frequencies fA1 and fB1 of the respective lights seen from the measured object 113 are indicated by the following equations:
                                          f            A1                    =                                                    V                A1                            λ                        =                          1              λ                                      ⁣                  ·                      (                          c              -                              V                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                α                                      )                          ⁢                                  ⁢                              f            B1                    =                                                    V                B1                            λ                        =                          1              λ                                      ⁣                  ·                      (                          c              +                              V                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                β                                      )                                              (        5        )            Relative velocities VA2 and VB2 of the respective scattered (reflected) lights and the measured object 113 are indicated by the following equations:VA2=c−V sin γVB2=c−V sin γ  (6)Consequently, frequencies fA2 and fB2 of lights seen from the observation point are indicated by the following equations:
                                          f            A2                    =                                                    c                                  V                  A2                                            ·                              f                A1                                      =                                          c                λ                            ·                                                1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    α                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                      ⁢                                  ⁢                              f            B2                    =                                                    c                                  V                  B2                                            ·                              f                B1                                      =                                          c                λ                            ·                                                1                  +                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    β                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                                          (        7        )            The difference between the frequency in Equation (7) and the frequency of incident light (f0) becomes a Doppler frequency shift quantity fd. Here, the beat frequency of the two light fluxes measured at the observation point 2fd is indicated by the following equation using c>>V:
                                                                        2                ⁢                                  f                  d                                            =                                                                                    f                    B2                                    -                                      f                    A1                                                                                                                                          =                                                V                  λ                                ·                                  (                                                            sin                      ⁢                                                                                          ⁢                      α                                        +                                          sin                      ⁢                                                                                          ⁢                      β                                                        )                                                                                        (        8        )            It can be seen that 2fd is independent of a position of the observation point (angle: Υ). In FIG. 13, in which α=β=θ is valid, the following equation is established based on Equation (8) in the typical optical system of the LDV of FIG. 13:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·            sin                    ⁢                                          ⁢          θ                                    (        9        )            Accordingly, the speed V of the measured object 113 is obtained by measuring the frequency fd indicated in Equation (3) and calculating using Equation (9).
It is also possible to interpret Equation (9) geometrically as follows: FIG. 15 is an enlarged view of an area in which the two light fluxes in FIG. 13 (the first and second light fluxes 107 and 108) intersect each other. The two light fluxes have an incident angle θ respectively and intersect each other, and the broken lines in FIG. 15 show parts of equal wave surfaces of the respective light fluxes. An interval between the broken lines shows the wavelength of light λ. Vertical thick lines show the bright parts of interference fringes, and when the interval between the vertical thick lines is set as Δ, this Δ is indicated by the following Equation (10):
                    Δ        =                  λ                      2            ⁢            sin            ⁢                                                  ⁢            θ                                              (        10        )            
As shown in FIG. 15, when an object (shown as •) passes perpendicularly to the interference fringes at a velocity of V, a frequency f of the interference fringes is indicated by the following equation:
                    f        =                              V            Δ                    =                                                                                          2                    ⁢                    V                                    λ                                ·                sin                            ⁢                                                          ⁢              θ                        =                          2              ⁢                              f                d                                                                        (        11        )            This equation is equal to Equation (9). This concept is called the interference fringe model.
The speed V of a measured object is thus obtained in a typical LDV. However, it is impossible to detect the moving direction of a measured object. On the other hand, detecting a moving direction is made possible by rotating the diffraction grating 103 in FIG. 13 at a velocity of Vg according to JP 03-235060 A. In this case, when light is reflected by the diffraction grating 103, since the respective light fluxes are Doppler frequency-shifted in proportion to Vg, the beat frequency 2fd to be measured in the PD 102 is obtained by the following equation:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·                          (                              V                +                                  V                  g                                            )                        ·            sin                    ⁢                                          ⁢          θ                                    (        12        )            Consequently, a moving direction of the measured object is obtained since the magnitude relation of 2fd is determined according to the positive and negative signs of the speed V with respect to the given velocity of Vg. According to the abovementioned optical system, however, a rotating mechanism for the diffraction grating 103 is required with result that the LDV becomes larger in size and higher in cost. In addition, the rotational speed of the diffraction grating 103 needs to be precisely maintained. However, it is difficult to employ the LDV for precise measurement due to problems such as an error caused by eccentricity, vibration and so on caused by rotation.
A velocimeter which solves the above problems is disclosed in JP 04-204104 A. In JP 04-204104 A, a moving direction of a measured object is detected by using a frequency shifter to change a frequency of an incident light flux.
FIG. 16 shows a schematic view of an optical system of a velocimeter disclosed in JP 04-204104 A.
According to the velocimeter, a light emitted from a laser light source 101 becomes a parallel light flux by a CL 104, and then are split into two light fluxes by a beam splitter (hereinafter, referred to as BS) 109. The respective light fluxes are reflected by mirrors 105 and are then frequency-shifted by f1 and f2 by an acousto-optic device (hereinafter, referred to as AOM) 110. The light fluxes are again collected on a surface of a measured object 113 by a diffraction grating 103 and then a beat frequency of scattered light from the measured object 113 is detected using a PD 102. The frequency 2fd to be detected here is indicated by the following equation:
                              2          ⁢                      f            d                          =                              (                                                                          f                  1                                -                                  f                  2                                                                    )                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        13        )            Consequently, a moving direction of the measured object 113 is detected by a magnitude relation of 2fd with respect to a known frequency shift quantity |f1-f2| since the sign of V changes according to the moving direction of the measured object 113.
Also in JP 08-15435 A, frequency is changed using an electro-optical device (hereinafter, referred to as EOM) 111 shown in FIG. 17 based on the same principle as that of JP 04-204104 A. More specifically, light emitted from a LD 101, which is a laser light source, becomes a parallel light flux by a CL 104, and is then split into two light fluxes comprised of a first light flux 107 and a second light flux 108 by a diffraction grating 103. The first and second light fluxes 107 and 108 are respectively made incident on the EOMs 111. Here, bias is applied to the second light flux 108 to shift frequency by fR. The first and second light fluxes 107 and 108 are respectively reflected by mirrors 105, and then are collected on a surface of the measured object 113. The beat frequency of lights scattered from the surface of the measured object 113 is detected using a PD 102. The frequency 2fd to be detected here is indicated by the following equation:
                              2          ⁢                      f            d                          =                              f            R                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        14        )            Thus, similarly to Equation (13), the moving direction of the measured object 113 is detected by the magnitude relation of 2fd with respect to a known frequency shift quantity fR since the sign of V changes according to the moving direction of the object.
However, an optical system where the moving direction of the measured object 113 is detected using frequency shifters such as the AOM 110 and the EOM 111, is disadvantageous in that the optical system is more complex and made larger in size since a power source for driving the frequency shifters is required so as to produce about tens of volts necessary to frequency-modulate by the AOM 110 or about 100 volts necessary to frequency-modulate by the EOM 111, for example, with the result that a large-sized power source is required.
Requests for miniaturization and lower power consumption concerning various sensors including the LDV have increased more and more, and this tendency is particularly strong for consumer products. Since the LDV detects scattered light, signal light from a measured object is generally weak although its strength may be different according to a type of the measured object. One solution is to employ a photomultiplier tube as a photodetector having high photosensitivity. However, when the photomultiplier tube is employed for the LDV, it causes the LDV itself to be large-sized. That is, the LDV including a photomultiplier tube is not suitable for application to small-sized consumer products. Instead, a photodiode, which is inferior as a photodetector in photosensitivity, is generally employed, so as not to obstruct miniaturization thereof. In this case, as much signal light as possible is preferably made incident on the photodetector. However, there is a limitation on a light reception system simply being disposed closer to the measured object since the distance between the light scattering surface of the measured object 113 and a condenser lens 106 is usually limited due to factors such as arrangement of optical components. As another measure for as much signal light as possible to be made incident on the photodetector, it is also possible to increase incident light quantity by employing gas lasers and so on of He-Ne and Ar+ as a high-power laser source, but a semiconductor laser is preferred from the viewpoint of device miniaturization and lower power consumption. In an LDV, a travel direction of a measured object is specified and is limited to a one-dimensional detection. Therefore, in order to obtain a two-dimensional velocity by an LDV, it is required that two-dimensional velocity is calculated based on velocities obtained from the two LDVs arranged in two different directions. When the two LDVs are thus employed, it is not practical not only because there is a possibility that an error is caused by the rotation, expansion and contraction of a measured object due to the different detection points, but also because space and cost for two devices are required.